1. Field of the Invention
The present invention is directed to a method of removing water from a silicon wafer or other substrate surface without the attendent formation of watermarks. More specifically, the present invention is directed to a method of removing water from the surface of a silicon wafer or other substrate by means of an inert gas stream in an atmosphere saturated with water vapor to suppress evaporative drying so that the liquid water is physically removed from the surface of the wafer without the attendant formation of watermarks.
2. Background of the Invention
The production of semiconductor devices include the processing of silicon or other semiconductor material wafers which serve as the semiconductor chip. Amongst the processing steps to which silicon wafers are subjected is at least one wet processing step which is terminated with a rinsing step with deionized water or other aqueous fluid. The rinsed water must be removed from the surface of the wafer in order to insure that the surface is free of moisture and chemical/particulate impurities or contaminants to provide optimal performance of the semiconductor device formed upon the silicon wafer.
The seemingly simple step of removing the liquid rinse water is anything but simple. This processing step has been a persistent and continuing problem in the processing of semiconductor chips. Specifically, the rinse water removal step has been plagued by the formation of xe2x80x9cwatermarksxe2x80x9d on the wafer surface. Suffice it to say, the retention of watermarks on the surface of silicon wafers adversely affects semiconductor device performance.
The xe2x80x9cwatermarksxe2x80x9d on the surface of silicon wafers are primarily the result of evaporative concentration of silica that has been etched from hydrophobic silicon areas of the wafer surface during water rinsing.
Etching occurs in pure, neutral (pH 7) water due to the natural ionization of water, yet becomes highly suppressed by adding enough acid to bring the pH down as little as two units (pH 5).
Acidification is one strategy to suppress unintended etching of Si in water with at best mixed results. Silicon etching is suppressed upon introduction of the acid; however, the acid also causes any previously formed soluble silicates (e.g., H2SiO3) to become less soluble colloidal silica, thus causing watermark formation. One attempt to use acid as a watermark prevention agent involved the introduction of carbon dioxide gas into a centrifugal drying process. The control of resulting carbonic acid on the wafer surface is problematic. Thus, the process requires very careful control and/or leads to very mixed results.
Other methods of addressing the aforementioned problem have been proposed. For example, U.S. Pat. No. 4,984,597 teaches the use of a drying vapor, preferably isopropanol, in conjunction with an inert carrier gas, to effectuate water removal from the surface of the silicon wafer. As those skilled in the art are aware, isopropanol is acidic. Thus, the above remarks regarding the detrimental effects of acidic conditions suggests the adverse consequences of employing isopropanol or other like alcohols in this application. It is moreover emphasized that isopropanol employed in the ""597 patent is present as a vapor. Isopropanol, however, is a liquid at ambient temperature and pressure. As such, isopropanol must also be subsequently removed from the wafer surface, entailing a further processing step. Yet another disadvantage of employing isopropanol is that isopropanol and other like alcohols are flammable and thus present environmental and safety problems.
U.S. Pat. No. 5,571,337 describes a process in which selected portions of a wafer surface is treated with an inert carrier gas or a mixture of a gaseous solvent and an inert carrier gas to remove liquid accumulated thereon. The process of the ""337 patent requires careful control of the water removal process which is generally impractical.
European Patent Application 0 385 536 describes the utilization of an organic solvent vapor miscible with liquid accumulated on the surface of the silicon wafer. This organic vapor is again preferably isopropanol. The theory driving the process of the ""536 application is that isopropanol vapor, when mixed with the liquid on the surface of the wafer, yields a product whose surface tension is lower than the surface tension of the liquid on the wafer surface. This difference in surface tension results in a gradient in the film of the liquid which causes a force to be exerted on the liquid film in the direction of a liquid bath. This phenomena is well known in that art as the Marangoni effect. The aforementioned discussion of the disadvantages of using isopropanol explains the disadvantages of this approach.
The above discussion explains that current silicon wafer drying techniques suffer from significant failings. Thus, the art is in need of a new process for removal of water from the surface of a silicon wafer or other substrate that is safe, does not require further processing steps and, most importantly, results in the production of silicon wafers or other substrates free of watermarks.
A new method has now been developed for the removal of surface water from the surface of a silicon wafer or other substrate. This method overcomes problems of incomplete removal of surface water, detrimental effects of fluids employed in removing water and introduction of acidic or hydroxyl-containing species enhancing chemical reactions resulting in the formation of watermarks.
In accordance with the present invention a method is provided wherein water present on a surface of a silicon wafer or other substrate is removed by disposing the wafer in an atmosphere saturated with water vapor whereupon liquid water and/or any aqueous solutions is physically removed from the surface of the wafer by a water saturated gas stream, more preferably, a water saturated inert gas stream. Thereupon, the last monolayer of water is removed by evaporation.